Condensation phase-change heat exchangers (i.e. condensers) are widely employed in fields of refrigeration, air conditioning, power generation, petrifaction, etc., due to high efficiency of phase-change heat transfer. In some applications, for example, in a case of an ORC cycle drived by a low-grade heat source including waste heat, solar thermal energy and geothermal energy, etc, the temperature and pressure in a condenser shall be lowered as much as possible result in small exergy loss and high efficiency, yielding a good system performance to achieve large power output from the thermodynamic cycle point of view, so that the condenser works under a small temperature difference (small temperature difference between an organic working medium in the tube and air or cooling water outside the tube), as a result, the heat exchange area and the investment cost are increased. Meanwhile, in fields of refrigeration, air conditioning, petrifaction, etc., further improving the condensation heat exchange efficiency may greatly reduce the cost, thereby having significant economic and social benefits. This provides significant requirements on the design, manufacturing and operation of a high-efficiency condenser.
Condensation phase-change is an important process in the two-phase flow science. Volume fraction of gas and liquid gradually change during the continuous evolution of the condensation process from a vapor state to a liquid state in the tube, so that different flow patterns, such as wet vapor flow, annular flow, stratified flow, slug flow, plug flow and bubble flow, present during the process. Meanwhile, due to continuous transform and accumulation of the condensate, small liquid droplets, a thin liquid film, a thick liquid film, a liquid bridge and finally a full-liquid state form in sequence. As the reported of studies, the annular flow in the state of a thin liquid film has the highest heat transfer efficiency, that is because the thin condensate film increases the heat exchange coefficient between vapor and solid. During the whole condensation, as the flow pattern is turned into the stratified flow, slug flow and plug flow from the annular flow, the liquid film with a certain heat resistance is gathered and become thicker and thicker even the liquid bridge state, the heat transfer resistance is obviously increased, so that the heat transfer coefficient is gradually decreased and the heat transfer efficiency is obviously deteriorated during the condensation. Therefore, the formation of thick liquid film during the condensation is the primary cause of the deterioration and attenuation of the heat exchange efficiency of the condensation tube.
At present, the conventional method for the condensation heat transfer enhancement use various types of enhanced tubes, for example, micro-fin tubes, groove tubes, corrugated tubes, and enhanced tubes with inserts. In the aspect of the enhancing effect, a micro-fin tube is generally more obvious by strengthening the mixing of the condensate film and resulting in disturbance of fluid close to the wall in the tube, so that the condensation heat transfer coefficient of a smooth tube may be improved by 80% to 180%. But for a groove tube with different oblique angles, the enhancing effect of the groove tube depends on the speed of the mass flow, that is, the greater the speed of the mass flow is, the quicker the export of the condensate, so the enhancing effect is more obvious. An enhanced corrugated tube generally may improve the heat transfer coefficient of a smooth tube by 50%. In addition, an enhanced tube with inserted a dual-helical filament structure may also obviously enhance the condensation heat exchange performance. However, for condensation enhanced tubes used at present, the heat transfer coefficient can be increased by mixing the fluid boundary layers and also by limiting the growth of fluid boundary layers close to the heat transfer surfaces, but no attention is paid to the modulation of the condensation flow patterns, and no design is made on the basis of the evolution of flow patterns. Those enhanced tubes have the following common points. (1) A fine structure of inner wall mainly changes the flowing and heat transfer performance in a near-wall region, incapability of regulating the flow patterns as a whole. (2) Although with an enhancing effect, the enhanced structure of inner wall fails to change the deteriorated evolution of heat transfer performance along the direction of length of the condensation tube. (3) The enhanced tube increases the difficulty of manufacturing, so the cost of a condenser is increased.
In 2007, Professor Peng Xiaofeng (Technical Principle and Practices of High-performance Condensers, Peng Xiaofeng, Wu Di, Zhang Yang, Chemical Industry and Engineering Progress, 2007, 26(1):97-104.), of Tsinghua University point out that the condensation heat transfer performance deteriorates along the length of the heat exchange tube with the assumption of a thin liquid film formed in the whole condensation tube according to Nusselt laminar flow and film condensation in which the liquid film thickness (δ) and average surface heat transfer coefficient (have) are direct proportion to L1/4 and E−1/4 respectively, where L is the tube length, Thus he suggested a high efficiency condensation tube with the short-tube effect, yielding the subsequent low-heat-transfer flow pattern of the condensation in the heat transfer tube is abandoned, while the initial high-heat-transfer flow pattern is remained. in addition, the method of feeding the vapor phase obtained by vapor-liquid separation using the gravity at the outlet of the short tube into the next short tube for further condensation to always maintain the condensation flow pattern as the annular flow obviously improves the heat transfer efficiency of the condensation heat transefer tube. The heat transfer performance is enhanced based on the mastering the heat transfer characteristic of condensation flow patterns and the science process of condensation. However, the consideration enhancement method of this document is to directly avoid flow patterns with poor heat transfer efficiency and shorten the length of the heat exchange tube. Meanwhile, the method of separating vapor from liquid by the gravity requires different design of a condenser for heat exchangers with different oblique angles, so the application has a certain limitation under microgravity.
In conclusion, obviously improving the condensation heat transfer performance and fundamentally solving the deterioration along with the tube length must regulate the flow pattern based on understanding of the mechanism of condensation process. The invention provides a novel internal liquid separating hood-type condensation heat exchange tube, different from the technique disclosed by Professor Peng Xiaofeng, according to features of thickened liquid film and increased heat resistance during the evolution of condensation, in which the condensate is timely guided and separated, thereby obtaining a high-efficiency condensation heat exchange tube capable of regulating the flow pattern and fundamentally improving the condensation heat exchange efficiency.